Pelletizing dies of the aforementioned type may have an axial length of 0.15-0.25 m and a diameter of about 0.4-0.6 m and may be fashioned as cylindrical elements of steel having a material thickness of about 0.5-0.15 m provided in a radial direction with a large amount of drilled through holes or passages having a diameter of about 1.3-10 mm.
Generally the holes or passages are drilled, from the periphery of cylindrical elements by a precisely guided drilling machine, with the holes or passages being distributed so that the holes are mutually spaced, with the spacing between the holes or passages being about two to three times their diameters, whereby the holes or passages, having their axes radially oriented, are distributed along several an annular rings encircling the cylindrical periphery of the die elements, for example, a hundred holes may be contained within each of twenty-forty-five annular ring formations encircling the die elements. The dies are employed for pelletizing, most often in the manner, that two or three cylindrical rolls at a distance of about 0.5-1 mm are arranged rolling against an internal surface of the cylindrical dies. A material to be pelletized, for example, a feed stuff powder, is supplied to the spacing thus arranged between the rolls and the inner surface of the die while they roll against each other. By application in this manner of a great pressure on the feed stuff powder, the powder is pressed through the single die holes or passages and is shaped into thin bars, which, by suitable cutting means are cut into suitable lengths at the outside of the die. With a die of the aforementioned size, which, for example, comprises 10,000 holes, each having a diameter of about 3 mm, five to ten tons of feed stuff may pass through the die per hour.
Experience shows that, in the course of time, unavoidably more and more of the holes or passages become clogged resulting in a reduction in the production and resulting in unwanted down time in the production, whereby the production down time may cause further clogging of the holes or passages in the dies to take place. Much of the clogging of the holes or passages is caused by small pieces of metal which, in one way or another, are present in the feed stuff mixture. However, partially clogged peripheral portions of the dies may have been caused by utilizing erroneous feed stuff mixtures or through an undesirable wearing of counter-sunk input ends of the die holes, which counter sinking was provided during the manufacturing of the dies.
Clogging may also be the result of the deforming cf the feed stuff input ends of the holes by the pressing rolls because of application of a too large pressure by the pressing rolls when the pressing rolls during manufacture of the feed stuff pellets, are being pressed against the inner side of the dies, etc. Dies causing reduced production have to be exchanged with dies giving optimum production. As the dies are rather expensive, it is desirable to restore or repair the dies.
Up to the present time, this restoring of the dies has been accomplished by a simple drilling process taking place by means of manually guided drilling machines, since this manner of restoring the dies is more efficient than using a pillar drill because pillar drills leave more broken drills unremovable stuck in the die holes as compared with the result when using a manual restoration process.
When employing the dies for pelletizing, the individual holes are worn down in a way which results in the holes being conically shaped having the conical shape with its largest diameter at the feed stuff material input end of the holes or passages. While the manual drilling process is rather slow, since the dies are expensive, it is less expensive to effect manual drilling operations than to replace the dies. In this connection, it should be noted that a problem exists in avoiding a deteriorating of the shape of the holes during a restoring or repairing of the dies since it is important to get the internal surfaces of the holes or passages as even as possible. This has shown itself to be difficult. When putting dies into operation, the suitable hole or passage polishing means is first often passed through the dies, or a feed stuff mixture with such properties is at first forced through the die holes or passages when attempting to restore or repair the dies.
To clean the dies, a manual washing has been tried, but, the washing water must be applied with so strong a force that is required to effect manual washing, that it is not possible to control the water application nor, the force applied to the washing water jet nor to achieve a noticeable washing by only simple means.
Experience has, on the other hand, shown, that if very large water pressures are used, for example, greater than 400 bar, then it is possible to achieve or rather to force an effective cleaning of such clogged holes or passages so that the net result is that only a few or practically no holes or passages remain clogged after a cleaning in this manner.
To achieve a sufficiently inexpensive restoration of such clogged dies, a high degree of mechanization has to be provided for because it, in practice, is not possible, with simple means as through manually or visually guiding of the water jet to obtain a satisfactory positioning of and also satisfactory direction of such a powerful water jet towards the hole or passage to be cleaned to achieve a cleaning of the holes or passages to take place.
Moreover, as short a time span as possible has to be spent for carrying out of the positioning of the water jet to be directed towards the hole or passage to be cleaned. Moreover, such water jet has a tendency of its own to positionally by itself to become or to remain locked somewhere along its path when trying to move it. This evidently is due to the fact that such a type of water jet in this case exhibits the same inherent property as that which is known from an upwardly directed broad jet of water, which is able to carry on its top, as rotationally shaped objects or bodies, such as evenly shaped ball members without loosing them from the jet. It must be admitted that the first trials of handling such a powerful water jet were more or less carried out as experiments without at first realizing the practical results, but, further experiments including trials with fast positionings and with suitably fast position adjusting of the water jet direction and, in general, by one step right-position-acquiring of the water jet, did indicate that it in someway was possible, without doing any damage due to the water jet to the hardened steel material of the die, to move or to displace a thin water jet of this powerful type from one hole or passage into a neighboring hole or passage for a cleaning of the holes or passages.
If a displacing of the jet is made too short, then the water jet exhibits a tendency to be locked in position at the middle of the material wall between the two holes or passages. On the other hand, if the new hole or passage to be cleaned is made to be hit or impinged upon by the water jet using sufficiently fast speed of displacement then, the water jet will lock at the center of the hole or passage. This locking is due to the effect that water returning from the spot being met by the approaching water jet at the displacement operation and later when the displacement is being finished. It has later been found that a cleaning is achieved when the water jet of the above mentioned pressure has a suitable small diameter, that is somewhat less than the diameter of the hole or passage, with an optimum for the water jet diameter being approximately one-half the diameter of the hole or passage. The cleaning water jet thus has to keep itself locked to the hole or passage which is being cleaned, also in the case that this hole or passage only is able to be cleaned by means of the water jet for a fraction of its length. On the other hand, a very high water pressure has to be present. Holes or passages which are clogged with feed stuff are cleaned fairly quickly, with the feed stuff leaving the holes or passages in a bullet-like manner. Severely clogged holes may require a longer time, for example, several seconds with such water jet treatment before a complete cleaning is attained. On the other hand it must be remembered that the number of holes or passages is very large.
Experience shows that the thin cutting jet apparently does not damage the die material, On the other hand, the energy which is supplied to the water jet and to the maintaining of the water jet is considerable and, for example, 50-75 kw are required for a water jet of 1-3 mm in diameter, resulting in a powerful heating of the water is taking place. Due to the heating of the water, the water. before possible reuse first has to be led to a suitable storage tank and necessarily has to be cooled.